Ultrasonic-assisted extraction of flavonoids from Camellia fascicularis leaves was optimized using response surface methodology. The optimal extracting conditions of flavonoids were determined to be the ratio of liquid to raw material of 60 mL/g, ethanol concentration of 40%, extraction temperature of 72.3°C, and extraction time of 1.6 h, which contributed to the highest flavonoids yield of 4.765%. The crude flavonoids were purified through an AB-8 macroporous adsorption resin column, eluted with ethanol concentration of 40%, and purified flavonoids were obtained. In vitro antioxidant assay demonstrated that purified flavonoids showed significant antioxidant capacities in a concentration-dependent manner for scavenging hydroxyl radical, superoxide anion radical, 2,2-diphenyl-1-picrylhydrazyl radical, and azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt radical. Overall, ultrasonic-assisted extraction was found to be an effective method of extracting flavonoids from C. fascicularis leaves, which might be explored as potential natural antioxidant.